Continuously variable valve timing mechanisms to control either intake valves, exhaust valves, or both, depending on operating conditions of the engine have been known. One known variable timing mechanism is a variable valve timing mechanism that continuously displaces the rotary phase difference (displacement angle) of a camshaft relative to the crankshaft.
On an engine equipped with such a variable valve timing mechanism, the actual rotary phase difference (actual displacement angle) is made to converge toward a target rotary phase determined from operating conditions of the engine using a feedback control system. Therefore, it is necessary for such an apparatus to accurately detect the actual displacement angle using a phase difference detection device as disclosed in Japanese Patent Application No. SHO 59-105911. This publication describes an apparatus for accurately detecting actual displacement angles by measuring a gap between a first pulse signal emitted at each rotation of the crankshaft and a second pulse signal emitted at each rotation of a camshaft.
If an abnormality occurs in the variable valve timing mechanism, e.g., when an odd object is trapped in the mechanism, it is impossible to make the actual displacement angle converge toward the target displacement angle, thus deteriorating the engine power output when various controlled parameters of the engine such as the timing of fuel injection are determined depending on the target displacement angle. Therefore, it is generally necessary to monitor whether any abnormality is occurring in a variable valve timing mechanism and to correct those controlled parameters to prevent the deterioration of the engine output when an abnormality occurs.
Abnormalities may be detected by comparing an absolute value of a difference between the target displacement angle and the actual displacement angle to a predetermined threshold value. When the absolute value of the difference exceeds the threshold value, an abnormality is determined to exist.
However, detecting abnormalities based solely on the comparison of the absolute value of the difference between the target displacement angle and the actual displacement angle to a predetermined threshold value tends to produce misjudgments. Such misjudgments may result because the absolute value of the deviation between the target displacement angle and the actual displacement angle may be quite large in an early stage of the control, as the actual displacement angle is displaced to follow the target displacement angle in that stage.
As a consequence, Japanese Patent Application No. HEI 8-232617 describes a device designed to avoid such erroneous detection of abnormalities. This apparatus attempts to avoid erroneous detection of abnormalities by setting up a judgment time in abnormality detection as it compares the actual displacement angle with the threshold value in detecting abnormalities.
A typical abnormality that can occur in a valve timing control apparatus is an abnormality caused by small metal particles that clog the valve timing control apparatus. However, such abnormalities sometimes disappear after a time through normal operation of the valve timing control apparatus without recourse to any remedial actions. For example, if the valve timing control apparatus uses hydraulic pressure for the purpose of control, such metal particles can be removed by the oil flow in the course of its natural operation.
A problem with the aforementioned apparatus is that abnormalities which may disappear in the course of natural operation of a valve timing control apparatus may be mistakenly identified as an actual problem, thus reducing the reliability of the abnormality detection apparatus.